Succinylacetone (SA), a potent specific inhibitor of heme biosynthesis, was shown to bind irreversibly to delta-aminolevulinic acid dehydrase, the second enzyme in the heme biosynthetic pathway. SA inhibits the growth of Walker carcinoma (W256) in vitro and in vivo, the Novikoff hepatoma in vivo, and L1210 leukemia in vitro, but only slightly in vivo. Since cellular heme levels are not decreased in W256 and L1210 cells, another unknown mechanism of growth inhibitionis functioning. The characteristics of heme uptake were examined in 3 malignant cell lines, a normal rat liver cell line, and a normal embryonic cell. The normal liver cell line, BRL-3A took up hematin at a slow rate while hematin uptake in murine erythroleukemia (MEL) cells, W256, L1210, and chick embryo fibroblasts was more rapid and of greater magnitude. Saturation of the heme uptake mechanism occurred at a much higher cellular heme concentration in L1210 cells than in MEL cells. Hematin uptake in MEL cells could be markedly stimulated by pretreatment of the cells with DMSO, procaine, detergent, or proteolytic enzymes or by increases in the pH of the medium from 8 to 9.5. SA caused a gradual increase in hematin uptake capacity in MEL cells over a period of days.